Any of the three you named would fly well with the 1.20. They are all about the same size and weight; it's really a matter of which one you like best (looks). The GP P-6 is a gorgeous plane with its distinctive markings, the Super Stearman is a nice aerobatic performer, and the WACO is........well it's a WACO, enough said. I have the 1/5 Cox WACO, and highly detailed it. I have a G-26 in mine; at 18 pounds, the G-26 is plenty of power and the plane performs very well. The basic ARF is about 14 pounds, so the 1.20 is going to be a good power match. There is a thread here on the ARF forum about the Cox 1/5 WACO and there are several engines being used. Look into it and see what others say.

One thing about all three planes you mention--you might need some nose weight with the glow engine to balance it. The people I know who have the P-6 and Super Stearman both have gas engines in theirs, and they balanced fairly well without extra weight. Even with the added weight, the 1.20 will be enough.

As far as quality, the Hawk and Stearman are both GP models and great quality. The WACO is by Cox, and less well known, but I feel it was about what you should expect, quality-wise, for what you're paying $380 for. I and some other owners are having a few issues, check out the Cox/Pica WACO thread.